This non-provisional application claims priority under 35 U.S.C. xc2xa7119 (a) on Patent Application No. 2001-174469 filed in Japan on Jun. 8, 2001, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to an exhaust emission purifying catalyst, and more particularly to an exhaust gas emission control catalyst with excellent durability and exhaust gas emission control capability.
2. Description of Related Art
In a lean combustion type engine such as a lean burn engine and a direct injection type engine, a three-way catalyst cannot exhibit satisfactory exhaust gas emission purifying performance during operation at a lean air-fuel ratio. To address this problem, a NOx occluding catalyst constructed such that a catalyst layer, to which is added an occluding agent, is supported on a cordierite carrier is installed in such an engine. However, in the NOx occluding catalyst, and more particularly in a catalyst with an alkali metal added thereto as an occluding agent, the occluding agent splashes or moves into the cordierite carrier at high temperatures to cause a decrease in the occluding agent. This deteriorates the NOx occluding performance of the catalyst.
Therefore, applicant of the present invention has proposed a catalyst to which zeolite is added so as to inhibit the occluding agent from splashing or moving into a carrier in Japanese Laid-Open Patent Publication (Kokai) No. 2001-129402.
However, if such a catalyst is operated at a very high temperature, the NOx occluding performance thereof is deteriorated thereafter. Therefore, there has been required a catalyst whose performance deterioration is small even after operation at a high temperature.
It is therefore an object of the present invention to provide an exhaust gas emission purifying catalyst which can withstand high temperatures.
To attain the above object, the present invention provides an exhaust gas emission purifying catalyst comprising: a carrier; and a catalyst layer to which at least one metal selected from a group consisting of alkali metals and alkali earth metals is added as an occluding agent; wherein a composite oxide including silicon (Si) and at least one metal selected from a transition metal group consisting of cobalt (Co), zirconium (Zr), iron (Fe), and manganese (Mn) is mixed in the catalyst layer.
Since the transition metals such as Co, Zr, Fe, and Mn have a high fusing point, the composite oxide of such transition metals and Si exhibit a high thermal stability. Thus, the occluding agent movement inhibiting capability of the Si in the composite oxide and the occluding function of the occluding agent are maintained even at high temperatures. Namely, the catalyst, in which is mixed the composite oxide comprised of the transition metals and the silicon, have a high capability to withstand heat, and thus, the excellent occluding performance of the catalyst can be maintained even after operation at a high temperature.